In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, dual connectivity communication has been specified (for example, see Non Patent Document 1). The dual connectivity communication is a communication mode in which a master cell group (MCG) and a secondary cell group (SCG) are set to a user terminal in an RRC (Radio Resource Control) connected mode. The MCG is a serving cell group managed by a maser base station. The SCG is a serving cell group managed by a secondary base station.